implement efficiency metrics

[XuhuiZhou]

solve #26

Tracks tokens (from vLLM TokenEvent IDs), steps, avg_tool_calls_per_step,
and wall_clock_duration

• New utility module: efficiency_metrics.py for computation
• Modified code_search_generator.py to capture timing and compute metrics
• Metrics saved to trajectory JSONs automatically during training
• Comprehensive test suite (19 tests, all passing)
• Backward compatible, works with both sync and async training modes

[lintangsutawika]

Made some adjustments:

1. Moved efficiency_metrics.py to src/metrics/
2. Adjust the parameters for the efficiency metrics that is returned from init_and_run as a dict.